C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  why prefix increment is faster than postfix increment? Goto page 1, 2, 3, 4, 5, 6, 7  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message jrefactors@hotmail.com Guest Posted: Sat Oct 22, 2005 8:01 am    Post subject: why prefix increment is faster than postfix increment? I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. i++ ++i Please advise. thanks!! Back to top Jonathan Mcdougall Guest Posted: Sat Oct 22, 2005 8:08 am    Post subject: Re: why prefix increment is faster than postfix increment? [email]jrefactors (AT) hotmail (DOT) com[/email] wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. i++ ++i http://www.parashift.com/c++-faq-lite/operator-overloading.html#faq-13.15 And while you're there, read the whole faq. Jonathan Back to top John Carson Guest Posted: Sat Oct 22, 2005 8:13 am    Post subject: Re: why prefix increment is faster than postfix increment? <jrefactors (AT) hotmail (DOT) com> wrote Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. They are obviously not both the same. The prefix form returns the value of the variable after the incrementation and the postfix form returns the value of the variable before the incrementation. For the postfix form, the simple way to implement it is for the compiler to produce code that creates a temporary that stores the original value and then to return this value. For a built-in type, this is a cheap operation. For a class object, it may be an expensive operation. -- John Carson Back to top Christian Bau Guest Posted: Sat Oct 22, 2005 9:19 am    Post subject: Re: why prefix increment is faster than postfix increment? In article <1129968097.386674.50960 (AT) g44g2000cwa (DOT) googlegroups.com>, [email]jrefactors (AT) hotmail (DOT) com[/email] wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. i++ ++i Please advise. thanks!! The C answer and the C++ answer for the built-in operators are: Whoever makes that claim is not only clueless, but is also the type of dangerous clueless person who thinks they have a clue. Don't take _any_ advice of them. Ever. I don't think you are interested in the C++ answer for operator overloading yet. Back to top Dave Rahardja Guest Posted: Sat Oct 22, 2005 2:25 pm    Post subject: Re: why prefix increment is faster than postfix increment? On 22 Oct 2005 01:01:37 -0700, [email]jrefactors (AT) hotmail (DOT) com[/email] wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. i++ ++i Please advise. thanks!! When i is a C++ object with the appropriate operators defined, the prefix version may eliminate the creation of a temporary object. -dr Back to top Kaz Kylheku Guest Posted: Sat Oct 22, 2005 4:17 pm    Post subject: Re: why prefix increment is faster than postfix increment? [email]jrefactors (AT) hotmail (DOT) com[/email] wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. i++ ++i If you are throwing away the result, there is no semantic difference at all, because the only difference between ++i and i++ is whether i or i + 1 is returned. If you don't throw away the result, then they are different operators; you can't substitute one without the other without making compensating changes in the surrounding program! You then end up with two different programs that you have to compare as such. If there is a performance difference between those programs, it's a result of not just changing the i++ to ++i, or vice versa, but also a consequence of those other compensating changes, and how your particular compiler and machine deals with everything as a whole. Note that in C++ (this is cross-posted to comp.lang.c++), both forms of the operator can be user-defined. If you are dealing with a choice between two forms of the user-defined operator, and performance is critical, you obviously have to take that into consideration! Back to top Gordon Burditt Guest Posted: Sat Oct 22, 2005 5:50 pm    Post subject: Re: why prefix increment is faster than postfix increment? Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are Any claim that X is faster than Y that doesn't specify a specific compiler and platform is FALSE. Even if Y is "do X 1000000 times". Gordon L. Burditt Back to top Dave Rahardja Guest Posted: Sun Oct 23, 2005 5:08 am    Post subject: Re: why prefix increment is faster than postfix increment? On Sat, 22 Oct 2005 17:50:01 -0000, [email]gordonb.foi6x (AT) burditt (DOT) org[/email] (Gordon Burditt) wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are Any claim that X is faster than Y that doesn't specify a specific compiler and platform is FALSE. Even if Y is "do X 1000000 times". I think the original poster heard one of those "rules of thumb" that aren't absolutely true or mathematically proven, but is considered a good approximation of the truth most of the time. I also assume that the OP wanted to know what the motivation was behind the saying. -dr Back to top Greg Guest Posted: Sun Oct 23, 2005 6:37 am    Post subject: Re: why prefix increment is faster than postfix increment? [email]jrefactors (AT) hotmail (DOT) com[/email] wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. i++ ++i Please advise. thanks!! Consider this program: void PrintElement(const std::vector<int>::iterator& i) { std::cout << *i << " "; } int main() { std::vector v.push_back(1); v.push_back(2); v.push_back(3); std::vector<int>::iterator i = v.begin(); while (i != v.end()) PrintElement( i++ ); while (i != v.begin()) PrintElement( --i ); } Which PrintElement() call is like the more efficient one: the one with the postincremented parameter (i++) or the pre-decremented (--i) parameter? Or is there no reason to think that there would be a difference? In this case, it is likely that first PrintElement call with the postfix incremented paramter has more overhead than the second, because the compiler must increment the iterator i before it calls PrintElement. But when the call to PrintElement is made, the compiler must pass the value of i (or a reference to a temporary copy of i) that i had before it was incremented. Therefore the compiler has little choice but to make a copy of i before incrementing i, so that it has an iterator with which it can call PrintElement. The second PrintElement call applies a prefix operator to the paramter; the compiler can therefore pass i directly to PrintElement, since its incremented value is the appropriate value to pass to PrintElement. Of course, the difference is not likely to be great, but there is nonetheless a basis for expecting postfix operators to be less efficient than prefix operators, especially when applied to parameters in a function call. Greg Back to top Martin Ambuhl Guest Posted: Sun Oct 23, 2005 7:20 am    Post subject: Re: why prefix increment is faster than postfix increment? Greg wrote: Quote: Consider this program: void PrintElement(const std::vector<int>::iterator& i) { std::cout << *i << " "; } [etc.] When you respond to posts which are crossposted to and <news:comp.lang.c++>, try to give answers that are acceptable in both. You have posted a bunch of compilation errors to <news:comp.lang.c>. There is no need to consider your code at all. Back to top Greg Guest Posted: Sun Oct 23, 2005 8:12 am    Post subject: Re: why prefix increment is faster than postfix increment? Gordon Burditt wrote: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are Any claim that X is faster than Y that doesn't specify a specific compiler and platform is FALSE. Even if Y is "do X 1000000 times". Not necessarily. If one can show that Y performs every operation that X performs, and then has to perform additional operations outside of that set and that require a measurable amount of time to complete, then one would have successfully proven that X is faster than Y. And in fact that is the case here: the postincrement operator may have to perform an additional copy operation that the prefix version does not have to perform. Otherwise the amount of work required of each is the same. Greg Back to top Michael Mair Guest Posted: Sun Oct 23, 2005 1:57 pm    Post subject: Re: why prefix increment is faster than postfix increment? Greg wrote: Quote: Gordon Burditt wrote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are Any claim that X is faster than Y that doesn't specify a specific compiler and platform is FALSE. Even if Y is "do X 1000000 times". Not necessarily. If one can show that Y performs every operation that X performs, and then has to perform additional operations outside of that set and that require a measurable amount of time to complete, then one would have successfully proven that X is faster than Y. And in fact that is the case here: the postincrement operator may have to perform an additional copy operation that the prefix version does not have to perform. Otherwise the amount of work required of each is the same. Nope. Maybe in source code but not necessarily in the generated code. The optimisation may have a stage which looks for certain patterns; it is very well possible that the seemingly slower code qualifies for the optimisation but the "faster" code does not. This may be the compiler's fault or yours, depending on the problem at hand. So, you are right most of the time but not always. However, C++ coding guidelines often bring the ++()/()++ including the caveats as an example of "avoiding premature pessimization" and rightly so. Cheers Michael -- E-Mail: Mine is an /at/ gmx /dot/ de address. Back to top Gordon Burditt Guest Posted: Sun Oct 23, 2005 4:10 pm    Post subject: Re: why prefix increment is faster than postfix increment? Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are Any claim that X is faster than Y that doesn't specify a specific compiler and platform is FALSE. Even if Y is "do X 1000000 times". Not necessarily. If one can show that Y performs every operation that X performs, and then has to perform additional operations outside of that set and that require a measurable amount of time to complete, then one would have successfully proven that X is faster than Y. Not unless you can prove that the compiler generates the code that way, and in order to do that, you have to choose a specific compiler. You cannot test *ALL* compilers, including ones written between your test and publishing the results. And you can't prove, for example, that the compiler doesn't add a time-wasting loop to X but not to Y, without referring to a specific compiler. There are some situations where doing more can be faster. For example repeating this statement 1000000 times could be faster than doing it 10 times: unsigned int x; x = x << 1; since if the width of x is less than 1000000 bits, this results in a constant independent of the original value of x, and the compiler might realize this, but if the width of x is greater than 10, and it must be, it has to shift x. Quote: And in fact that is the case here: the postincrement operator may have to perform an additional copy operation that the prefix version does not have to perform. Otherwise the amount of work required of each is the same. You're assuming things about the underlying instruction set that may not be true, and assuming that the compiler doesn't do a poor job of generating code for one and a good job for the other. The effect of a cache hit/miss can also do funny things to code that looks like it should run in the same time as other code. Gordon L. Burditt Back to top Christian Bau Guest Posted: Sun Oct 23, 2005 4:44 pm    Post subject: Re: why prefix increment is faster than postfix increment? In article <1130055121.143006.220940 (AT) f14g2000cwb (DOT) googlegroups.com>, "Greg" <greghe (AT) pacbell (DOT) net> wrote: Quote: Gordon Burditt wrote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are Any claim that X is faster than Y that doesn't specify a specific compiler and platform is FALSE. Even if Y is "do X 1000000 times". Not necessarily. If one can show that Y performs every operation that X performs, and then has to perform additional operations outside of that set and that require a measurable amount of time to complete, then one would have successfully proven that X is faster than Y. One would have proven no such thing. Consider this: double x, y; memcpy (&x, &y, sizeof (x) - 1); memcpy (&x, &y, sizeof (x)); A good compiler will translate the second memcpy to a simple assignment of a double variable, for example: Load double y into register Store register into double x. In fact, if this is the only time the address of x and y is taken, it is quite possible that x and y are still kept in floating-point registers, in which case this is a very fast register-to-register assignment. On the other hand, the first memcpy will have a much more difficult implementation, even though it copies one byte less. Not only will it produce much more code, on most current processors that code will execute considerably slower. Back to top Jack Klein Guest Posted: Sun Oct 23, 2005 8:01 pm    Post subject: Re: why prefix increment is faster than postfix increment? On 22 Oct 2005 01:01:37 -0700, [email]jrefactors (AT) hotmail (DOT) com[/email] wrote in comp.lang.c: Quote: I heard people saying prefix increment is faster than postfix incerement, but I don't know what's the difference. They both are i = i+1. What people are these? What are their credentials so that you, or we, should place any confidence in their opinions? Quote: i++ ++i Please advise. thanks!! One possible item of advice would be for you to associate with different people. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Goto page 1, 2, 3, 4, 5, 6, 7  Next Page 1 of 7   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.